Paints typically include colorants, such as pigments or dyes, dispersed in a binder. In many cases, the colorants are transparent or semi-transparent pigments, such as is often the case with certain reds, yellows and oranges. While such colorants are often desired, one problem that may arise from their use is that they may not adequately “hide” the substrate upon which the paint is deposited. This is often the case because the index of refraction of such colorants is similar to that of the binder in which the colorant is dispersed. As a result, such colorants scatter little or no light, so that light more easily reaches the substrate. To solve this hiding problem, the film thickness and/or the amount of colorant in the paint may be increased. Both of these solutions, however, can be costly because, at least in part, of the relatively high cost of colorants.
Another known way to improve the hiding ability of a paint composition is to include substrate-hiding materials, such as titanium dioxide, zinc oxide, and/or lead oxide, in the paint composition. Of these, titanium dioxide is most often used by paint manufacturers. These materials, sometimes referred to as “opacifying pigments,” usually have an optimal particle diameter of around 200 nanometers and a refractive index that is significantly higher than that of the binder in which the material is dispersed. As a result, they provide whiteness and are good hiding pigments.
The use of these white hiding pigments, however, has not been without drawbacks. Typically, particles of hiding pigments, such as titanium dioxide, are randomly dispersed in the binder along with the coloring pigment. To achieve sufficient hiding, however, sufficient levels of hiding pigment must be used so that there is a statistical probability that light will be scattered by particles of hiding pigment before reaching the substrate. As a result, a significant amount of hiding pigment may reside at or near the top surface of the coating film, away from the substrate. In such a case, a significant amount of light may be reflected by the white hiding pigment without ever passing through the colorant, thereby affecting the chromaticity, or purity, of the paint color.
It has been proposed to include in a coating a flake-like assemblage in which scattering pigments are encapsulated in a resinous polymer. U.S. Patent Application Publication No. 2003/0125417A1 discloses that when such assemblages are included in a paint composition and the paint is applied to a substrate, the assemblages tend to align with the substrate and with each other in parallel along their longest dimensions to create multidirectional scattering of light in the paint composition. The coatings of this application, however, are directed to the use of specially processed colorants having a primary particle size of less than about 150 nanometers dispersed in a resinous binder. Such colorants are, however, often undesireable because, for example, they require extra processing to produce, are costly, and they are less commercially available as compared to conventional colorants, which have an average particle diameter of greater than 150 nanometers. In addition, the use of colorants having a primary particle size of less than about 150 nanometers may be especially undesirable in certain applications because of metamerism effects. Metamerism is caused by the nature of human color vision and results from the fact that two colored articles may appear the same color under illumination with a given light source, even though they may have differences in their reflection spectra. Because of these differences, these same articles may appear different from each other in color under a different light source. This can be particularly problematic because, for example, a refinish paint composition could appear to match an original automotive paint when indoors but fail to match outdoors. Because most original automotive paints contain conventionally sized pigments as colorants, it is particularly desirable to provide refinish paint compositions that with good hiding and color purity that also use conventionally sized pigment particles.
Accordingly, there is a need for improved pigmented compositions that use conventionally sized transparent or semi-transparent colorants, and which can (i) exhibit improved hiding as compared to coatings that include such transparent or semi-transparent pigments in the absence of any substrate-hiding materials, and (ii) exhibit improved color purity as compared to coatings that include dispersed transparent or semi-transparent pigments and substrate-hidings materials.